1. Field of the Invention
The present invention pertains to a lyocell multi-filament for a tire cord, a method of producing the same, the tire cord and a tire for an automobile using the same. More particularly, the present invention relates to a lyocell multi-filament for a tire cord, which has excellent physical properties for the tire cord, thereby providing a tire with improved driving stability, dimensional stability, and uniformity for an automobile, a method of producing the same, the tire cord and a tire for an automobile using the same.
2. Description of the Related Art
As well known to those skilled in the art, a material of a tire cord used as a framework constituting a tire is selected from the group consisting of polyester, nylon, aramid, rayon, and steel. In this regard, it is required that the material of the tire cord has the following excellent physical properties: 1) high strength and initial modulus, 2) excellent heat resistance, and no degradation under dry and wet heat, 3) excellent fatigue resistance, 4) excellent dimensional stability, 5) excellent adhesiveness to rubbers (refer to Fukuhara, Fiber & Industry, 1980, Vol. 36, pp 290). However, the above materials cannot have all the above excellent physical properties as described above, so the material of the tire cord depends on the intended use of the tire cord.
For example, a radial tire, requiring excellent initial modulus (elasticity), heat resistance, and dimensional stability, for high-speed driving of an automobile comprises a tire cord mostly consisting of a rayon fiber with low shrinkage and excellent dimensional stability. At this time, the initial modulus is usually expressed as load per unit stretch for a certain fiber denier, in other words, as a slope of an elongation-load curve in a strength and elongation test. The higher the initial modulus of the tire cord is, the less the tire will be deformed, so the high initial modulus contributes to improving fatigue resistance, heat resistance, and durability of the tire. Particularly, the high initial modulus improves transverse-strength of the radial tire, thus excellent driving stability of the radial tire is secured. Additionally, the rayon tire cord has excellent driving stability in comparison with various tire cords consisting of other materials because its physical properties are rarely degraded at a temperature of 80 to 100° C. during driving of the automobile.
However, the rayon tire cord has relatively low tenacity and its modulus is greatly degraded by moisture, so it is difficult to control moisture and quality of the tire during the production of the tire including the rayon tire cord. Additionally, even if the tire including the rayon tire cord is manufactured, when a surface of the tire is damaged and moisture penetrates into the damaged tire, strength and modulus of the tire are reduced, thus being poor in terms of its performance. Accordingly, there is a need to develop a tire cord having excellent strength and modulus against moisture, in addition to having excellent tenacity.
Meanwhile, an artificial lyocell fiber consisting of cellulose is advantageous in that elongation is low and tenacity is high, so its dimensional stability is excellent, and its strength preservation proportion is 80% or higher when the lyocell fiber absorbs water because of its low moisture regain. Accordingly, in comparison with rayon (60%), the lyocell fiber is competitive in terms of low reduction of modulus and low deformation. However, the lyocell fiber has not been used as the tire cord because of the spinning related problems.
A commercial value of fibers used in the tire cord or other industrial materials depends on their physical properties such as tenacity and modulus while the commercial value of fibers for clothes depends on dyeability for vivid or bright colors and ease of care.
Accordingly, each textile maker continuously improves each textile's qualities using various fiber production technologies according to use of the fibers. Various technologies have been developed to improve physical properties of the fibers. For example, when molecular chains constituting a polymer are desirably oriented along a fiber axis, the fibers have excellent physical properties useful to be applied to clothes and various industrial fields. In this regard, orientation is conducted during the drafting process, so the drafting process is one of the most important processes capable of improving physical properties of the fibers.
Furthermore, the drafting process is conducted under a thermoplastic state in which fluidity of molecules is good according to a melt-spinning process. Additionally, according to a solution-spinning process, after a solution including three components, that is, a solvent, a non-solvent, and a polymer is prepared, the solution is spun using a wet spinning or dry spinning method. The drafting process is conducted while vaporizing the solvent in the case of the dry spinning method, but in the case of the wet spinning methods, drafting of the fibers is conducted during the coagulation process, so depending on a concentration and temperature of a coagulation liquid.
Further, in the case of producing the lyocell fiber, when a solution including NMMO (N-methyl morpholine N-oxide), water, and cellulose at relatively high temperature of 80 to 130° C. is spun in such a way that a spinning nozzle is dipped in a coagulation bath according to the traditional wet spinning method, the solution is too quickly coagulated to secure desirable physical properties. Additionally, it is difficult to sufficiently vaporize the solvent from a high viscous cellulose solution of about 10000 poises using only the dry spinning method.
Meanwhile, a dry-wet method may be used to improve physical properties of the fibers and spinning efficiency by properly utilizing air gaps positioned between the spinning nozzle and the coagulation bath.
For example, EP. Pat. A-259,672 discloses a process of producing an aramid fiber, in which the drafting and coagulation process are conducted using air gaps to improve physical properties of the aramid fiber, and U.S. Pat. No. 4,501,886 suggests a process of spinning cellulose triacetate using air gaps. Additionally, Japanese Pat. No. 81,723 by Mitsubishi Rayon Co. describes a high-speed spinning process of a PAN (polyacrylonitrile) fiber using air gaps, East German Pat. No. 218,124 discloses a process of spinning a cellulose solution using a tertiary aminoxide-based aqueous solution, in which air gaps are used to prevent a plurality of filaments from adhering to each other, and U.S. Pat. No. 4,261,943 discloses a process of spraying water acting as a non-solvent to air gaps each having a space of 50 to 300 mm to prevent a plurality of filaments from adhering to each other.
The processes as described above contribute to improving orientation of the fibers using the air gaps. However, they are not useful to be directly applied to the production of a lyocell multi-filament, because filaments are apt to adhere to each other because of a great number of filaments, so desired spinning efficiency is not obtained. As well, the lyocell fiber produced by the above processes has inadequate tenacity and elongation for use as a tire cord.
Further, H. Chanzy et al. (Polymer, 1990 Vol.31, pp 400-405) propose a process of producing a fiber using air gaps, in which salts such as ammonium chloride or calcium chloride are added to a solution of cellulose with the degree of polymerization (DPw) of 5000 in NMMO and the resulting mixture is then spun to produce the fiber with tenacity of 56.7 cN/tex and elongation at break of 4%. However, it is difficult to commercialize this process because of various disadvantages, for example the recovery of the coagulation solution containing salts.
Further, U.S. Pat. No. 5,942,327 describes a process of producing a fiber with tenacity of 50 to 80 cN/tex, elongation of 6 to 25%, and monofilament fineness of 1.5 dtex using air gaps, in which a solution of cellulose with the degree of polymerization (DPw) of 1360 in NMMO hydrate is spun. At this time, the number of filaments of the resulting fiber is just 50 filaments. In general, the filament for a tire cord must have fineness of about 1000 deniers, so hundreds of plies of filaments are needed to secure fineness of about 1000 deniers. Accordingly, this patent is disadvantageous in that it is difficult to secure the tire cord with desired physical properties after twisting or dipping. Practically, it is difficult to control spinning conditions of quenching in the air-gap, washing, and drying of the fiber during spinning of the fiber with large denier in comparison with the spinning process of the fiber with small denier, so rarely securing desired physical properties of the fiber and scarcely maintaining uniformity of the filaments. Accordingly, it is nearly impossible to produce the industrial fiber referring to physical properties of the fiber with 50 filaments. Furthermore, a process of spinning the solution to the air gaps requires a new design accompanying with additional considerations, such as an outer diameter of the spinning nozzle, a diameter of an orifice, intervals between orifices, a length of each of the air gaps, feeding conditions of cooling air, and a drying condition of the filaments depending on a feeding direction of the coagulation liquid and a spinning speed, because adherence of the filaments to each other and quenching efficiency are varied according to an increase of the number of the filaments. In this regard, physical properties of the fiber depend on the design.
Moreover, U.S. Pat. No. 5,252,284 discloses a process of spinning a fiber under conditions of air gaps each having a length within about 10 mm and a winding speed of 45 m/min to produce the fiber consisting of 800 to 1900 filaments. However, this patent is disadvantageous in that elongation is a relatively high 15.4% and tenacity is at most 47.8 cN/tex, thus securing insufficient competitiveness of the fiber for use as a tire cord in terms of tenacity and productivity.
Additionally, some methods of producing a mixture solution of cellulose and polymer using NMMO are known in the art.
For example, U.S. Pat. No. 3,447,939 discloses a process of producing a solution containing cellulose and polyvinyl alcohol dissolved in NMMO, and U.S. Pat. No. 3,508,941 proposes a method of dissolving a mixture of cellulose and polyvinyl alcohol in NMMO to extract the mixture. Further, according to U.S. Pat. No. 4,255,300, when a mixing ratio of cellulose and polyvinyl alcohol is 4:1 to 2:1 and a percent composition ratio of a polymer to a solvent is 20% or lower, a fiber has excellent elongation. However, U.S. Pat. No. 4,255,300 does not disclose the fact that the tenacity of the fiber is improved because polyvinyl alcohol is added to cellulose.
Meanwhile, U.S. Pat. No. 6,245,837 discloses a process of producing a fiber with a tenacity of 27 cN/tex, in which a mixture including cellulose, polyethylene, polyethylene glycol, polymethylmethacrylate, and polyacrylamide is dissolved in a NMMO solution. However, this patent is disadvantageous in that the fiber has very poor tenacity to be used as an industrial filament or a tire cord.
Therefore, there remains a need to develop a cellulose solution for a high strength cellulose filament.
The present inventors have made an effort to develop the cellulose solution for the high strength cellulose filament, and found the fact that a cellulose/polyvinyl alcohol/NMMO solution suppresses the generation of fibril while a cellulose fiber is formed and the cellulose fiber having excellent flexibility and tenacity can be produced using the cellulose/polyvinyl alcohol/NMMO solution, thus the cellulose/polyvinyl alcohol/NMMO solution is usefully applied to an industrial filament or a tire cord.
Furthermore, the present inventors have conducted extensive studies into the method of producing a lyocell filament useful as a tire cord, resulting in the finding that the lyocell multi-filament for tire cords with excellent physical properties can be obtained by providing a method of producing the lyocell multi-filament, comprising the steps of dissolving mixed powder of cellulose and polyvinyl alcohol in a mixed solvent of N-methyl morpholine N-oxide and water to prepare a dope, extruding the dope using a spinning nozzle including orifices through air gaps into a conical upper solidifying bath to solidify the dope to produce a multi-filament, feeding the multi-filament through a lower coagulation bath to a washing bath, washing multi-filament, drying and oiling multi-filament, and winding the resulting multi-filament, thereby accomplishing the present invention.